It is standard practice to send batches of different fluids in the same pipeline, for example, different crude oils or gasolines, diesel fuel, kerosene, and furnace oil. Gravitometers which continuously weigh small fixed volumes of the fluids flowing in the pipeline are utilized to detect the interface in the pipeline between different fluid batches. Difficulties arise where the specific gravities of adjacent batches are similar, which is not uncommon, for example similar fluids from different shippers, or two brands of gasoline. Accordingly, in such instances, it is desirable to have some interface detection means available which does not rely upon specific gravities.
Further difficulties may arise in the use of gravitometers when it is desirable not only to detect the interface between batches, but also to identify the oil or other fluid comprising a batch. Impurities in the batch such as water can prevent correlations between the specific gravity of the batch and the identifying specific gravity of the pure fluid of the batch.
Prior art considered pertinent to the present invention includes U.S. Pat. Nos. 3,546,926 and 3,189,180 and co-pending application Ser. No. 905,261, filed May 12, 1978 now U.S. Pat. No. 4,184,952.